1. Field of Invention
This invention relates to improvements in refractory compositions for flow casting.
Conventional castable refractories which have been generally used for flow casting, contain about 15 parts by weight of alumina cement as a main binder which may reduce heat- and corrosion-resistance of the refractories due to its relatively high CaO contents, and also cause explosion by rapid drying because of liberating water of crystallization when heated.
Some of the present inventors have developed a useful refractory composition for flow casting in which such alumina cement is not used as a main binder. Such composition is disclosed in Japanese patent application No. 52-40699, corresponding to U.S. Pat. No. 4,171,984.
In the composition of this prior invention, a combination of an alkali metal silicate and a sparingly water-soluble aluminum tripolyphosphate are used as a main binder, and a small amount thereof is added to the refractory material. The refractory composition of this prior invention exhibits enough flowability while being cast into the mold because of the slow hardening reaction of the sparingly water-soluble aluminum tripolyphosphate--which exhibits very low solubility in water at room temperature--to the alkali metal silicate, and then hardens after a period of time required for casting has passed.
This refractory composition has been used as a lining material for blast furnace guide troughs, which are, in general, used under such severe service conditions that conventional castable refractories can not be used, and good results have been obtained.
Furthermore, it exhibits better corrosion resistance to molten iron and slags than usual trough lining materials applied by ramming, and has the greater advantage that the life-time of the furnace material is extended remarkably so that consumption of the material per ton of produced iron can be reduced by half.
It also has the advantage in case of being applied to the wall of an ingot heating furnace, that no explosion is observed even by rapid drying, nor any heat spalling is caused while in use, and double life-time is expected when compared to the usual castable refractories.
However, such refractories of the prior invention still have the following defects. In application, refractory material of the prior invention is mixed with water and cast into a mold of desired shape and is kept standing for curing, but because of the slow hardening characteristics of sparingly water-soluble aluminum tripolyphosphate, enough green strength, i.e. shape retention, can not be obtained by such curing at room temperature, so that when the mold is removed, the cast material moves with the mold and as a result, the surface of the material deforms or small cracks occur within the body, and cause constructional defects to reduce the life-time of the furnace.
Therefore, such insufficient green strength may prevent quick removal of the mold after casting when required as for example in connection with the furnace operation, defects appear more often when the mold is removed in less than 10 hours after casting and such defects may increase with the thickness of the work and become remarkable when the thickness exceeds about 300 mm. Therefore, in practice, the work is heated from outside of the mold before removing the same, but this may require troublesome and time wasting operations.